1. Field of the Invention
The present invention generally relates to a brake control system which controls a braking force applied to a wheel to prevent the wheel from being locked during braking of the vehicle. More particularly, the present invention relates to a technique for initiating brake control at a proper timing.
2. Description of the Related Art
During braking of a vehicle, a gripping force of a tire with respect to a road surface is lessened due to a braking force applied to the vehicle, and rotation of the wheel almost stops. This is called a brake lock or a lock. When the wheel is locked, the vehicle becomes undirected, so that the vehicle cannot be steered effectively, or a vehicle body collapses.
To prevent the wheel from being locked, anti-lock brake control (hereinafter referred to as ABS control) is known, which controls a braking force applied to the wheel. This ABS control is as follows. If a slip ratio of the wheel or its change increases during braking and it is determined that the wheel is more likely to be locked, the ABS control is initiated. In the case of a motorcycle, it is determined whether or not to initiate the ABS control based on a slip ratio derived from a difference between a front wheel speed and a rear wheel speed. In some cases, the ABS control cannot be initiated at a proper timing, due to low calculation accuracy of the slip ratio.
During deceleration of the motorcycle, a slip ratio of the wheel (front or rear wheel) with a lower wheel speed with respect to the wheel with a higher wheel speed is derived. If the rear wheel is slightly spaced apart from a road surface in rapid deceleration, an incorrect slip ratio may be calculated.
To solve this problem, in the ABS control disclosed in Japanese Laid Open Patent Application Publication No. 2007-296908, to prevent calculation of the incorrect slip ratio due to an event in which the rear wheel is spaced apart from the road surface, the slip ratio is calculated based on a parameter other than the rear wheel speed in a predetermined state in the deceleration.
However, the above stated prior art example is effective in a particular situation in which the rear wheel is slightly spaced apart from the road surface during rapid deceleration of the motorcycle, and cannot improve overall calculation accuracy of the slip ratio during braking.
To make up for insufficient calculation accuracy of the slip ratio, it may be determined whether or not to initiate the ABS control, based on a magnitude of a braking force applied to the wheel such as a hydraulic brake pressure. However, the magnitude of the braking force indirectly indicates that the wheel is more likely to be locked, unlike the slip ratio. Therefore, it would be difficult to determine whether or not to initiate the ABS control at a proper timing, based on the magnitude of the braking force.